Two-stroke internal combustion engine and cylinder head for the latter

ABSTRACT

A two-stroke internal combustion engine, in particular of the diesel type, which comprises at least one intake valve (10) having its seat disposed in the wall of a combustion and scavenging prechamber (13) and at least one exhaust valve (7), is characterized in that the prechamber (13) communicates with the cylinder (1) through a transfer passageway (14) whose walls are at least partially substantially parallel to the axis (2) of the cylinder and whose cross-section perpendicular to this axis opens out in accordance with a substantially oblong shape tangent to the cylinder (1). An improved effectiveness of the engine is obtained.

The present invention generally relates to a two-stroke internalcombustion engine having at least one cylinder containing areciprocating piston, in particular but not exclusively of the dieseltype, and it more particularly concerns a valve device exclusivelyincorporated in the cylinder head which permits the replacement of theburnt gases by fresh air required for the combustion.

The invention also relates to a cylinder head for internal combustionengines which is provided with said device and to the variousapplications and utilizations resulting from its use.

The replacement of the burnt gases by the charge of fresh air presents aparticular problem in two-stroke internal combustion engines, sincethere is only a short period of time (corresponding to an angle ofrotation of about 120° to 140° of the crankshaft) for achieving it,whereas, in four-stroke engines, the lapse of time available for thispurpose is substantially longer and may correspond to an angle ofrotation of about 400° of the crankshaft.

In two-stroke engines having modern valves, one tries to improve thescavenging:

(a) by increasing the permeability of the work chamber or cylinder whenthe intake and exhaust valves are simultaneously open;

(b) by decreasing the short-circuit between the intake and the exhaustby means of the orientation of the current of particles of fresh airentering the cylinder, in a direction which prevents them from passingdirectly from the intake to the exhaust,

(c) by reducing as far as possible the mixture, in the cylinder, betweenthe fresh air and the burnt gases coming from the preceding cycle orcycles.

In U.S. Pat. No. 2,061,157 (Hurum), it has already been proposed todispose, in the cylinder head of a two-stroke internal combustionengine, one or two intake valves which open onto a prechamber of flatshape and whose axis or axes are orthogonal to the axis of the cylinder,and an exhaust valve whose axis is parallel to that of the cylinder andoffset relative to the last-mentioned axis. The prechamber communicateswith the cylinder through an orifice of restricted section so as tocause the mixture of air and fuel to enter the cylinder in the form of acompact jet and the stem of the or each intake valve extends through thespace defined in the cylinder head by the geometrical extension of thewall of the cylinder, which gives rise to a throttled and dissymmetricalflow of the mixture into the cylinder. Experience has shown that saidcompact jet was not very effective from the scavenging point of view:indeed, if the criterion (b) is respected, owing to the introduction athigh velocity of particles of air in the cylinder toward the piston, onthe other hand, the criterion (c) is not respected: the introduction ofparticles of air at high velocity in the cylinder is effected, owing tothe disposition of the intake valve, in the very midst of the gasmass--particularly at the moment when the intake valve is at thebeginning of the opening--and creates an intense mixture of the freshair and burnt gases. Further, this disposition leaves dead regions whichare not scavenged, which still further reduces the scavengingeffectiveness. Owing to the presence of the orifice of restrictedsection, the permeability of the cylinder head to the flow (criterion a)is very poor. Lastly, the flat shape of the prechamber results in a badmixture between the fresh air and the fuel which is injected thereinto.This analysis of U.S. Pat. No. 2,061,157 is confirmed in the thirdparagraph of U.S. Pat. No. 4,616,605 (Kline) which was only published onOct. 14, 1986.

In U.S. Pat. No. 2,222,134 (Augustine) there is described a two-strokeinternal combustion engine having an intake and an exhaust valve whoseaxes are parallel to that of the cylinder and whose opening movementsare in opposite directions The seat of the intake valve opens upwardlyonto the prechamber which opens downwardly onto the cylinder through anorifice in the shape of a crescent disposed tangentially to thecylinder. The geometry of the prechamber is such that a high turbulenceoccurs around the intake valve and causes a disorientation of theparticles of fresh air entering the cylinder, producing a large shortcircuit (criterion b) not respected) and that the air is preferentiallydirected into the extrados of the elbow connecting the prechamber to thecylinder, which will cause the particles of air to enter in the verymidst of the gas mass, resulting in a large mixture of fresh air withthe burnt gases (criterion c) not respected).

An object of the invention is to improve the operation of a two-strokeinternal combustion engine, in particular but not exclusively of thediesel type, having at least one cylinder with a reciprocating pistonand a device for exchanging the gases which is achieved exclusively byat least one intake valve and at least one exhaust valve disposed in thecylinder head at the top of the associated cylinder, so as to obtain ascavenging which respects all three criteria defined above.

The invention has therefore principally for an object, in an engine ofthe aforementioned type, to increase the effectiveness of the exchangeof the gases, i.e. to expel as far as possible the residual burnt gasesfrom the cylinder by replacing them by a corresponding volume of freshair, while preventing or at least reducing as far as possible any riskof a direct passage of the fresh air from the intake valve to theexhaust valve and simultaneously avoiding as far as possible anycreation of a region of a mixture of fresh air and burnt gases, with aminimum expenditure of energy. The expense of energy is minimized by thesearch for the best possible utilization of the scavenging air suppliedto the cylinder, as described before, but also by the obtainment ofgreater permeability, i.e. by the realization of maximum flow sectionsoffered to the gaseous fluids thus requiring a minimum pressuredifference between the pressure of scavenging air and the back-pressurein the exhaust to ensure a given scavenging air flow. The effectivenessof the exchange of the gases of the two-stroke internal combustionengine is thus characterized by the quality of the utilization of thescavenging air, on one hand, and by the permeability of the cylinder onthe other. These two characteristics directly condition the power andthe efficiency of the cycle of the diesel engine which is notsupercharged and also, but to a lesser degree, of the diesel enginewhich is moderately or highly supercharged.

All the observations made before for diesel engines apply to engineswhich have a controlled ignition or a natural aspiration or aresupercharged.

In the case where, for these engines, the preparation of the preferablyhomogeneous mixture of air and fuel is effected upstream of the cylinderby means of a carburetor or a fuel-injection system, it becomesnecessary to obtain an exchange of the gases without a short-circuit ofcarburetted fresh air to the exhaust. In two-stroke engines having aloop scavenging through intake and exhaust ports, it is thusconventional to undergo losses of air, and therefore fuel, of up to 30%and even 40% of the fresh charge retained in the cylinder in the courseof the exchange of the gases, which has a correspondingly adverse effecton the fuel consumption. The geometry of the structure must moreoverpermit a satisfactory combustion, which is in practice manifested by thenecessity of simultaneously satisfying antagonistic conditions orrequirements. The object of the invention is therefore to realize acompromise between a good efficiency of the scavenging and of thecombustion with the simplest technology while as far as possibleretaining the aforementioned advantages and reducing thepreviously-mentioned drawbacks.

In order to facilitate the following description, it will be assumedthat the position of the cylinder is such that its axis is vertical andthat the cylinder head occupies the upper or top position and the pistonthe lower or bottom position.

The present invention solves the aforementioned technical problems byproviding a two-stroke internal combustion engine having at least onecylinder with a reciprocating piston and a device for exchanging gasesentirely incorporated in the cylinder head and comprising a group of atleast one intake valve and a group of at least one exhaust valve, eachintake valve having its seat disposed in the wall of a combustion andscavenging prechamber, said device exchanging the gases having a planeof symmetry passing through the axis of the cylinder and common to thedisposition of the group of at least one intake valve, to thedisposition of the group of at least one exhaust valve, and to theconfiguration of the interior surface of the prechamber and of the roofof the cylinder head and to the configuration of the surface of thepiston, characterized in that the prechamber communicates with thecylinder through a transfer passageway whose walls are at least partlysubstantially parallel to the axis of the cylinder and whose crosssection perpendicular to this axis opens, according to a substantiallyoblong shape tangential to the cylinder, and the or each intake valvecooperates with the lateral wall of the prechamber practically withoutclearance with the latter in the upper part of said valve so that thecircuit of intake air, upstream of the valve, opens directly onto thetransfer passageway downstream of the valve, including during the firstinstants of the rising of the valve.

According to another characteristic of the invention, the axis of eachintake valve has a direction which is not parallel to the direction ofthe axis of the cylinder and makes with the latter an angle preferablybetween about 45° and about 90°.

According to yet another feature of the invention, the seat associatedwith each intake valve is located in a wall portion of the prechamberextending at least approximately the wall portion of the transferpassageway tangent to the surface of the cylinder.

According to a first embodiment, a single intake valve and a singleexhaust valve are provided.

According to a particular embodiment, the gas exchange device has twointake valves parallel to each other.

According to another particular embodiment, the gas exchange device hastwo exhaust valves parallel to the axis of the cylinder. According toanother characteristic of the invention, the engine is characterised inthat the cross section of the passageway opening onto the cylinder isdeveloped in a circular sector having an angle subtended at the centreof between 60° and 110° and represents an area representing a ratiorelative to that of the cross section of the cylinder of preferablybetween 0.10 and 0.20 and more particularly between 0.13 and 0.17.

According to another embodiment of the invention, the bottom wall of thescavenging and combustion prechamber substantially opposed to thetransfer passageway opening onto the cylinder is constituted by aportion of a cylinder of revolution coaxial with each intake valve,substantially tangent to each valve head, so that the radial clearancebetween said wall and the head of each intake valve has a minimum valuewhich is such that each intake valve discharges directly and essentiallyon its sector oriented in the direction of the transfer passageway so asto orient the quasi-totality of the air flow issuing from each intakevalve directly toward the transfer passageway.

According to yet another embodiment of the invention, the engine ischaracterized in that the radial clearance is as small as possiblebetween the upper part of each intake valve and the lateral andcylindrical wall coaxial with the corresponding valve, of the prechamberin the angular sector substantially opposed to the transfer passageway.

According to another of its aspects, the invention also concerns acylinder head of two-stroke internal combustion engines arranged inaccordance with the previously-explained characteristics.

The invention affords, among others, the following important advantages:

It permits rendering the sections of the intake and exhaust valvesmaximum while requiring only a relatively slight deviation of the meanstream of air during its passage from the intake manifold to theinterior of the cylinder, which results in a substantially increasedpermeability as compared with other solutions employing the same numberof intake and exhaust valves.

The scavenging effectiveness is improved, since it permits, whileensuring a high permeability, the obtainment of a high scavengingefficiency with a very good utilization of the scavenging air, whilereducing as far as possible any risk of a direct passage of fresh airfrom the cylinder to the exhaust valve, owing to the confinement of thestream which is accelerated toward the piston without being able todeviate in the direction of the exhaust valve, including during thefirst instants of the opening of the intake valve.

The experimental development for the obtainment of a good scavengingeffectiveness is considerably simplified owing to the small number ofparameters governing the formation of the air stream. Indeed, for thefirst part of the rising of the valve, and therefore at a low scavengingflow, the shape of the walls defining the prechamber and leading to thetransfer passageway is preponderant for the orientation of the airstream onto the wall of the liner which is the most remote from theexhaust valve while with increasing rise and high scavenging flow, thisfunction is performed in major part by the shape of the tulip of theintake valve and of the associated seat, enabling the flow to effect abend at about 90° between the intake pipe and the cylinder with aminimum of throttling after the passage through the neck.

Apart from the obvious constructional simplification afforded by theinvention in the variant using only a single intake valve, a singleintake valve forbids any dissymmetry of the scavenging air streamrelative to its previously defined plane of symmetry, which is alwaysdifficult to avoid when there are for example two intake valves owing toa possible evolution in operation of their respective clearance or oftheir respective soiled state. The important participation of thegeometry of the transfer passageway, which represents a fixed geometryas opposed to the essentially variable geometry of the intake valve, inthe formation of the scavenging air stream, permits the realization of ascavenging air stream of great stability in all cases of load andrunning speed of the engine. The transfer passageway contributes, as thecase may be, to the re-establishment of an improved symmetry of thegaseous stream.

The invention ensures a successive scavenging of the prechamber and thecylinder so that, even in the case of a very small quantity ofscavenging air, the volume of the prechamber is scavenged and filledalmost exclusively with fresh air before the compression stroke (asopposed to nonscavenged combustion prechambers). This has forconsequence that, in the described extreme case corresponding tooperation with a partial load, the volume of comburent air is in theupper part of the prechamber after having been urged back by theresidual gases coming from the cylinder during the compression stroke.

There are thus created, somewhat by a stratification effect, conditionswhich are to be very advantageously exploited for controlling theoperation of the engine under minimum load, whether it concerns anengine having a compression ignition or an engine having a controlledignition. In both cases, the means for introducing fuel (injector)and/or for ignition will be preferably placed in the part of theprechamber opposed to the seat of the intake valve.

The movement of the piston at the end of the rising travel of thelatter, i.e. in the vicinity of its upper dead centre, causes thetransfer of the charge of fresh air from the cylinder to the prechamberand thus creates a field of turbulence which is all the more intense asthe dead space is small between the head of the piston, which ispreferably flat, and the inner end of the cylinder head where the headof the exhaust valve is flush in the closed state.

The turbulence prevailing in the combustion prechamber at the moment ofthe injection of the fuel, in the period immediately preceding the upperdead centre position of the piston, may be strongly influenced by theresidual turbulence of the vortex issuing from the scavenging phase inthe direction opposed to the turbulence field created by the rising ofthe piston.

The fact that the combustion and scavenging prechamber is both scavengedand cooled by the scavenging air and the major part of the heat givenoff in the course of the combustion phase occurs in said prechamber,permits containing the thermal charge of the cylinder head and of theupper part of the cylinder while equalizing the highest temperatures ofthe constituent parts of the cylinder head and of the cylinder exposedto the combustion gases. This advantage is preponderant for a two-strokeengine in which it is well known that the thermal charge is higher thanin the case of a four-stroke engine and this, more particularly inrespect of engines employing very high maximum cycle pressures (forexample on the order of 200 to 300 bars) as envisaged within the scopeof the invention.

The disposition and the size of the intake and exhaust valves permit theuse of the inside of their seats for providing in the known manner anannular cooling passageway to ensure the cooling of said valves but alsothat of the cylinder head proper, owing to the very large fraction ofthe surface of the cylinder head in contact with the combustion gaseswhich are thus naturally irrigated by the cooling water of said valveseats.

The horizontal or inclined disposition of the intake valve permits theactuation thereof by a very direct drive, in particular by a lateralcamshaft disposed in the upper part of the engine block, in the case ofmulticylinder engines provided with individual cylinder heads, or by anoverhead camshaft in the case of engines having a single cylinder head.This configuration permits, owing to the small masses in motion, therealization of very high acceleration values when the intake valve isopened and closed, without exceeding the allowable limits of contactpressure in the region of the cam, which is very favorable since theopening diagram of the intake valve is very short (on the order of 100°to 140° of rotation of the crankshaft) and shorter than that of theexhaust valve (on the order of 20° to 40° of rotation of thecrankshaft). This disposition favours the realization of intake valverises which are greater than that which is conventional in known engines(the ratio between the maximum rise and the inside diameter of the seatof the valve may reach and exceed twice the normal ratio) forcompensating the fact that the intake valve only discharges in its lowerpart bearing in mind its practically zero radial clearance in its upperpart, with the lateral surface of the prechamber opposed to the transferpassageway.

This permits an elegant solution of the aforementioned problem presentedby the very short period of opening of the valves and in particular theintake valves.

All of the advantages described hereinbefore and principally relating tothe effectiveness of the scavenging and of the combustion permit therealization of an excellent scavenging and combustion efficiency up tohigh values of the stroke/bore ratio of the cylinder, in particularhigher than the values known elsewhere for scavenging in a loop or in acorner achieved by means of ports disposed in the liner or exclusivelyby means of valves disposed in the cylinder head. The obtainment of agood scavenging efficiency with a very high stroke/bore ratio (up to 2and even 2.5) is in keeping in a very favorable manner with the actualtechnical evolution of large-bore diesel engines for naval or groundapplications, since the search for the highest efficiency today resultsin the use of ever-increasing stroke/bore ratios (making it possible toobtain a high volumetric compression ratio and a high combustionefficiency) on the order of 3 to 4 for slow two-stroke engines having aunicurrent scavenging and a cross construction and 1.5 to 2 forsemi-rapid four-stoke engines in both cases to the detriment of theweight and the overall size. This characteristic indeed permits thecontemplation of the application of the device according to theinvention to semi-rapid two-stroke engines having a scavengingexclusively through the cylinder head, which, owing to the knownadvantages of the two-stroke cycle as concerns specific power, wouldcontribute for these high efficiency engines, which are however ofincreasingly large size, to a substantial improvement in theweight/power ratio (on the order of 30% for an unchanged efficiency)

Lastly, the geometrical configuration of the scavenging and combustionprechamber provides very high volumetric ratios which may reach and evenexceed 20, this being true also in the case of stroke/bore ratios closeto unity. This fact facilitates the starting up conditions of dieselengines of very small size, for example, in the automobile application.

A better understanding of the invention will be had and other objects,characteristics, details and advantages thereof will appear more clearlyin the course of the following description with reference to theaccompanying diagrammatic drawings, in which:

FIG. 1 is a fragmentary view, in cross-section, only of the elementsrelating to the invention, i.e. of the head of a cylinder and of theassociated cylinder head portion of a two-stroke diesel engine having adistribution through an intake valve and an exhaust valve which areperpendicular to each other and are both represented open during thescavenging and filling stage in the vicinity of the bottom dead centreof the piston;

FIG. 2 is a horizontal cross-sectional view taken on line II--II of FIG.1, showing the opening of the transfer passageway onto the cylinder;

FIG. 3 is a sectional view taken on line III--III of FIG. 1;

FIG. 4 is a sectional view similar to that of FIG. 2 of an embodimenthaving two intake valves which are parallel to each other;

FIG. 5 is a sectional view similar to that of FIG. 2 of an embodimenthaving two exhaust valves parallel to each other;

FIG. 6 represents, to an enlarged scale, a preferred variant of theembodiment of FIGS. 1 to 3;

FIG. 7 is a sectional view taken on line VII--VII of FIG. 4;

FIG. 8 represents the diagram of the opening periods of the intake andexhaust valves as a function of the angle of rotation of the crankshaft;

FIGS. 9a to 9h represent the different phases of the cycle of operationof the variant represented in FIGS. 1 and 2;

FIG. 10 represents another embodiment of the invention in a fragmentaryview similar to that of FIG. 1, in which the control of the valves by asingle common overhead camshaft is clearly shown.

According to the embodiment shown in FIG. 1, the reference 1 designatesa cylinder of a diesel engine having one or more cylinders operating inaccordance with a two-stroke cycle, having a geometric axis 2 hererepresented in a substantially vertical position and containing areciprocating piston 3 represented in a position close to its bottomdead center.

This cylinder 1, here for example constituted by a wet liner type, ismounted in the cylinder frame or block 4 of the engine and usuallysurrounded by a cooling water jacket 5. The upper end or head of thecylinder is surmounted and closed by a cylinder head 6 which contains anexhaust valve 7 controlling an exhaust pipe 8 for the burnt gasescommunicating with an exhaust line 9 forming in particular an exhaustmanifold, and an intake valve 10 controlling an intake pipe 11 for freshcomburent air communicating with an intake manifold 12. The intake valve10 and the intake pipe 11 open on to, in the direction of flow of thefresh scavenging air, a scavenging and combustion prechamber 13 which isformed in the cylinder head 6 and opens on to the cylinder 1 bycommunicating with the latter through a transfer passageway 14. Thedisposition of the intake valve 10 and exhaust valve 7 preferably allowsa plane of symmetry moreover corresponding to the plane of FIG. 1 andcontaining the axis of the exhaust valve 7, the axis of the intake valve10 and the axis 2 of the cylinder 1, the axis 2 being shown in dot-dashline in FIG. 1.

The axis of the exhaust valve 7 is substantially parallel to the axis 2of the cylinder and offset from the latter so that, in the openposition, the head of this exhaust valve 7 is located on one side (onthe left side of FIG. 1) relatively close to the correspondingneighbouring lateral wall of the cylinder 1 and on the other side (onthe right side of FIG. 1) relatively remote from the opening out of thetransfer passageway 14.

The axis of the intake valve 10 opening on to the prechamber 13 is notparallel and is here represented preferably at least orthogonal to thewalls of the cylinder 1 and therefore to the axis of the exhaust valve 7and to the axis 2 of the cylinder. As is clear from FIG. 1, the stem 17of the valve 10 extends away from this axis 2 in said plane of symmetry.

The exhaust valve 7 cooperates with a fixed seat 15 provided in thecylinder head 6. Likewise, the intake valve 10 cooperates with a fixedseat 16 provided in the cylinder head 6.

The transfer passageway 14 has a wall 14a at least partly substantiallyparallel to the axis 2 of the cylinder 1, the part 14b of the walllocated adjacent to the intake valve 10 in fact constituting anextension of the wall of the cylinder 1 (see FIG. 2). The opposite partof the wall 14a of the transfer passageway 14 in fact also constitutesan extension of the part of the wall 13a of the premixture chamber 13opposed to the intake valve 10. The transfer passageway 14 moreover hasin cross-section perpendicular to the axis 2 of the cylinder 1, asubstantially oblong shape tangent to the cylinder 1, as is clearlyshown in FIG. 2. The cross-section of the transfer passageway 14 openingon to the cylinder 1 is preferably developed on a circular sector havingan angle subtended at the centre of between 60° and 110° and representsan area whose ratio relative to that of the cross-section of thecylinder 1 is preferably between 0.10 and 0.20 and, more particularly,between 0.13 and 0.17.

In its upper part, the prechamber 13 has, from the seat 16 of the intakevalve 10, a cylindrical portion of revolution 18 coaxial with the intakevalve 10, substantially tangent to the head 10a of the valve 10 andhaving such dimension that there is practically no air flow in the upperpart of the head 10a of the intake valve 10. This cylindrical portion 18therefore constitutes in practice the top or the end wall of theprechamber 13.

Further, the wall part 14a of the transfer passageway 14 is connected tothe lower part of the valve seat 16 by an arcuate profile 22 permittinga direct flow of air to the transfer passageway 14 from the start of theopening of the intake valve 10.

As is clear in particular from FIG. 3, the cylindrical part ofrevolution 18 substantially coaxial with the intake valve 10 leavesbetween this wall 18 and the head 10a of the intake valve 10 a radialclearance 32 having a minimum value preventing the creation of asignificant air stream around the upper part of the head 10a of theintake valve 10. Consequently, the quasi-totality of the air flowissuing from the intake valve 10 flows around the lower part of the head10a of the intake valve 10 to the transfer passageway 14, assymbolically represented by the flow arrows 28 of FIG. 3.

With reference to FIG. 4, a second embodiment of the invention has beenshown according to which two intake valves are provided respectivelydesignated by 100 and 110, in the upper part of each of which there isprovided, as in the case of FIG. 3, a minimum radial clearance 32 whichis just sufficient for the passage of the heads of these valves. Asshown in FIG. 7, this permits the injection of fuel in theaforementioned plane of symmetry and also, as will be explainedhereinafter, deriving benefit from the organized turbulence produced bythe flow from the cylinder 1 resulting from the rising of the piston. Inthis embodiment, a single exhaust valve 7 is provided.

With reference to FIG. 5, there has further been shown an embodiment ofthe invention in which two exhaust valves designated respectively 107and 117 are provided, with a single intake valve 10.

In each of these embodiments of FIGS. 4 and 5, the single valve, namelythe exhaust valve 7 or the intake valve 10, is in the aforementionedplane of symmetry.

FIG. 8 represents the opening diagram of the intake and exhaust valvesof the preferred embodiment of FIGS. 1, 2 and 3. In the usual manner,the intake opening is designated 10, the exhaust opening is designatedEO, the intake closure IC, the exhaust closure EC, the top dead centreTDC and the bottom dead centre BDC.

The opening period of the exhaust valve 7 represents about 160° of theangle of rotation of the crankshaft, while the open period of the intakevalve 10 represents about 140° of the angle of rotation of thecrankshaft. It will be observed in this respect that the opening periodof the exhaust valve 7 starts well before the opening period of theintake valve 10, respectively 60° and 30° before the bottom dead centre.

With reference to FIGS. 9a to 9h, different sequences of the cycle ofoperation of this engine have been shown.

FIG. 9a represents the expansion phase in respect of which the intakevalve 10 and the exhaust valve 7 are closed and the piston 3 travelstoward the bottom dead centre as represented symbolically by the arrowF.

FIG. 9b represents the following sequence in respect of which theexhaust valve 7 has just opened while the intake valve 10 is stillclosed, the piston 3 continuing its downward movement toward the bottomdead centre, which will permit, as known per se, the lowering of thepressure in the cylinder 1 to the level of the scavenging pressure.

FIG. 9c represents the following sequence in respect of which theexhaust valve 7 is roughly completely open, the piston being at thebeginning of its upward stroke as shown by the inverted direction ofarrow F, while the intake valve 10 is already practically open and thuspermits the flow of the air stream which has been designated for exampleby 28 in FIG. 3.

This flow 28 is converted into a single air flow 40 bearing against thevertical wall of the cylinder following on the transfer passageway 14which discharges, as it enters the cylinder 1, a corresponding volume ofburnt gases 42.

FIG. 9d represents the following sequence corresponding to thescavenging of the cylinder 1 and showing the maximum rises of theexhaust valve 7 and the intake valve 10 respectively. Note in thisrespect that this maximum rise of the intake valve 10 is greater thanthat of usual two-stroke engines. As indeed is known, the rise of avalve is so calculated that the lateral area of the geometric cylinderlimited between the valve seat and the transverse surface of the valveis equal to or slightly greater than the free section of the open valveseat. In the case of the invention, it is only about one half of thelateral area of said geometric cylinder which allows the passage of thefresh air, and it is consequently necessary to compensate for this lossof area by increasing the rise of the intake valve 10 or of the intakevalves 100, 110. For this purpose, preferably the ratio between themaximum rise of the or each intake valve 10 and the inside diameter ofthe seat 16 of said intake valve exceeds 0.35.

It will be observed that, at this moment of the cycle, the intake airflow 40, which is practically without mixture with the burnt gases 42and is almost exclusively supplied by the stream 28 owing to theposition of the head 10a of the intake valve 10, occupies almost thewhole of the volume of the cylinder 1 and has urged back the major partof the burnt gases 42.

FIG. 9e represents the sequence of the end of the scavenging for whichthe exhaust valve 7 has just closed, the intake valve 10 being partlyopen before its complete closure. The piston 3 continues its upwardtravel in the cylinder 1 and urges back a part of the air in thedirection of the intake manifold 12.

FIG. 9f represents the following compression sequence for which the twovalves, namely the exhaust valve 7 and the intake valve 10, are closed.The continued upward travel of the piston in the cylinder therefore notonly produces the compression but also a progressive discharge of air tothe prechamber 13, which results in a large turbulence field.symbolically represented by the arrow 50, suitable for the fuelinjection phase and the mixture of the fuel with the comburent air inthe following sequence.

FIG. 9g represents the fuel injection phase just before the top deadcentre, symbolically represented by a fuel jet 52.

Lastly, FIG. 9h represents the last sequence relating to the combustionof the mixture thus prepared with the piston at its top dead centre.Owing to the described structure and to this operation, all thetechnical advantages mentioned in the introduction part of thedescription are obtained.

Moreover, it will be understood that various modifications may be madewithout however departing from the scope of the invention. The inventiontherefore comprises all the means constituting technical equivalents ofthe described means and their various combinations.

In particular, any usual means may be used in combination with the meansof the invention, whether this concerns the rocker arms, the design ofthe injection and of the combustion chamber, the design of the structureof the cylinder head which may be advantageously of the type known perse having bored passageways. Moreover, in FIG. 1, there have beendesignated by 70, 72 passageways for cooling the seats 15, 16 of theexhaust valve 7 and intake valve 10, which permits a cooling not only ofthe valves themselves but also of the major part of the cylinder head 6exposed to the combustion gases.

It has been seen in this respect in the introduction part of thedescription that it was possible to almost completely avoid the presenceof water chambers by imparting in this way to the cylinder head a veryhigh structural rigidity.

Further, according to another embodiment shown in FIG. 10 similar tothat of FIG. 1 and in respect of which the same reference charactershave been used for identical parts, it may be arranged that thedirection of the axis of the intake valve 10 make an angle equal toabout 50° with the direction of the axis 2 of the cylinder 1. In thiscase, there may be realized advantageously a control of each group of atleast one intake valve 10 and of each group of at least one exhaustvalve 7 by a single common overhead camshaft 150 acting on eachaforementioned valve through associated rocker arms 152, 154 providedwith rollers 156, 158, the valve-return means having been omitted inorder to render the drawing more understandable.

According to the variant of FIG. 6, the head 10a of the intake valve 10has an approximately planar surface 19 adapted to cooperate with aconjugate surface 20, also approximately planar, of the seat 16. Theopposite side 21 of the head 10a, which is preferably approximatelyconical, is so arranged as to enter a cavity 30 of conjugate shapeprovided in the opposite wall of the prechamber 13, the whole being suchthat the intake valve 10, at its maximum rise, practically fullypenetrates this cavity and expels the burnt gases. Further, the intakepipe 11 is advantageously provided with a lip 33, immediately upstreamof the seat 16 and on its lower part, adapted to progressivelyaccelerate, by a nozzle effect, the fresh air entering the prechamber 13upon the opening of the intake valve 10.

Again preferably, the fuel is introduced under pressure in theprechamber 13 through an injector 120 disposed, not at the top of thisprechamber as diagrammatically shown in FIG. 1, but on the axis of theintake valve 17, which improves the homogeneity of the mixture of airand fuel admitted to the cylinder. In the case where two symmetricalintake valves 100 and 110 exist (FIG. 7), there may be provided only asingle injector 120 which discharges along the axis of symmetry of theassembly of these two valves.

I claim:
 1. A two-stroke internal combustion engine having a cylinderhead (6), at least one cylinder (1) with a reciprocating piston (3), anda device in said cylinder head (6) for exchanging burnt gases with airand having a plane of symmetry containing an axis (2) of the cylinder(1),said device comprising a group of at least one intake valve (10) anda group of at least one exhaust valve (7), said intake valve having aseat (16) disposed in a wall of a scavenging and precombustion chamber(13), said device characterized in that the precombustion chamber (13)communicates with the cylinder (1) through a transfer passageway (14)having a wall (14a) at least partially substantially parallel to theaxis of the cylinder (1) and whose cross-section perpendicular to saidaxis opens out to form a substantially oblong shape tangentially to asurface of said cylinder and, said intake valve (10) cooperates,practically without clearance (32), with an end wall part (18) of saidprecombustion chamber (13) which is in a substantially opposedrelationship to said transfer passageway (14), and said device alsocharacterized in that a portion of the wall of the precombustion chamber(13) in which the seat (16) associated with said intake valve (10) islocated extends tangent to the surface of the cylinder (1).
 2. An engineaccording to claim 1 characterized in that said device comprises asingle intake valve (10) and a single exhaust valve (7).
 3. An engineaccording to claim 1, characterized in that the device for exchangingsaid gases has two intake valves (100, 110) which are parallel to eachother.
 4. An engine according to claim 1, characterized in that thedevice for exchanging said gases has two exhaust valves (107, 117)parallel to the axis of the cylinder.
 5. An engine according to claim 1,characterized in that a cross-section of the transfer passageway (14)where said transfer passageway opens onto the cylinder (1) is developedon a circular sector having an angle subtended at the centre of between60° and 110° and represents an area the ratio of which to that of across-section of the cylinder (1) is between 0.10 and 0.20.
 6. An engineaccording to claim 1, characterized in that the end wall part (18) ofsaid precombustion chamber (13) is constituted by a cylindrical portionof revolution (30) coaxial with said intake valve (10) and substantiallytangent to an associated valve head so that a radial clearance (32)between said end wall part (30) and a head of said intake valve (10) hasa minimum value which is such that said intake valve (10) dischargesdirectly and essentially on a sector thereof oriented in towards thetransfer passageway (14) so as to orient the quasi-totality of an airflow issuing from said intake valve (10) directly toward the transferpassageway (14).
 7. An engine according to claim 1, characterized inthat each group of at least one intake valve and each group of at leastone exhaust valve are controlled by a single common overhead camshaft.8. An engine according to claim 1, characterized in that a ratio betweena maximum rise of said intake vale (10) and an inside diameter of a seat(16) of said intake valve is greater than 0.35.
 9. An engine accordingto claim 1, characterized in that a head (10a) of said intake valve (10)cooperates, by its surface (21) remote from its seat (16), with a cavity(30) of conjugate shape provided in an opposed wall part of saidprecombustion chamber (13).
 10. An engine according to claim 1,characterized in that the seat (16) of said intake valve (10) is flatand tangent both to the wall of the transfer passageway (14) and thesurface of said cylinder (1).
 11. An engine according to claim 1,characterized in that an intake pipe (11) for said intake valve isprovided with a lip (33) immediately upstream of the seat (16) of saidintake valve and on a part of said pipe (11) closest to said transferpassageway.
 12. An engine according to claim 2, characterized in that afuel injector (120) opens onto the precombustion chamber (13)approximately on an axis of said intake valve (10) and on a side of saidprecombustion chamber opposite to said intake valve.
 13. An engineaccording to claim 1 wherein a stem of said intake valve is locatedcompletely outside of an extension of the cylinder when said intakevalve is closed.
 14. A two-stroke internal combustion engine having acylinder head (6), at least one cylinder (1) with a reciprocating piston(3), and a device in said cylinder head (6) for exchanging burnt gaseswith air and having a plane of symmetry containing an axis (2) of thecylinder (1),said device comprising a group of at least one intake valve(10) and a group of at least one exhaust valve (7), said intake valvehaving a seat (16) disposed in a wall of a scavenging and precombustionchamber (13), said device characterized in that the precombustionchamber (13) communicates with the cylinder (1) through a transferpassageway (14) having a wall (14a) at least partially substantiallyparallel to the axis of the cylinder (1) and whose cross-sectionperpendicular to said axis opens out to form a substantially oblongshape tangentially to a surface of said cylinder and, said intake valve(10) cooperates, practically without clearance (32), with an end wallpart (18) of said precombustion chamber (13) which is in a substantiallyopposed relationship to said transfer passageway (14), and said devicealso characterized in that a cross-section of the transfer passageway(14) where said transfer passageway opens onto the cylinder (1) isdeveloped on a circular sector having an angle subtended at the centreof between 60° and 110° and represents an area the ratio of which tothat of a cross-section of the cylinder (1) is between 0.10 and 0.20.15. An engine according to claim 14, characterized in that an axis ofeach said intake valve (10) has a direction which is not parallel to theaxis (2) of the cylinder (1) and makes with the latter axis and angle ofbetween about 45° and 90°.
 16. A device provided in a cylinder head of atwo-stroke engine and associated with a cylinder and piston of theengine for exchanging burnt gases with air,said device comprising agroup of at least one intake valve (10) and a group of at least oneexhaust valve (7), said intake valve having a seat (16) disposed in awall of a scavenging and precombustion chamber (13), said device havinga plane of symmetry containing an axis of the cylinder, said devicecharacterized in that the precombustion chamber (13) communicates withthe cylinder (1) through a transfer passageway (14) having a wall (14a)at least partially substantially parallel to the axis of the cylinder(1) and whose cross-section perpendicular to said axis opens out to forma substantially oblong shape tangentially to a surface of aid cylinderand, said intake valve (10) cooperates, practically without clearance(32), with an end wall part (18) of said precombustion chamber (13)which is in a substantially opposed relationship to said transferpassageway (14), and said device also characterized in that a portion ofthe wall of the precombustion chamber (13) in which the seat (16)associated with said intake valve (10) is located extends tangent to thesurface of the cylinder (1).
 17. An engine according to claim 16,characterized in that a cross-section of the transfer passageway (14)where said transfer passageway opens onto the cylinder (1) is developedon a circular sector having an angle subtended at the centre of between60° and 110° and represents an area the ratio of which to that of across-section of the cylinder (1) is between 0.10 and 0.20.
 18. Anengine according to claim 16, characterized in that the end wall part(18) of said precombustion chamber (13) is constituted by a cylindricalportion of revolution (30) coaxial with said intake valve (10) andsubstantially tangent to an associated valve head so that a radialclearance (32) between said end wall part (30) and a head of said intakevalve (10) has a minimum value which is such that said intake valve (10)discharges directly and essentially on a sector thereof oriented intowards the transfer passageway (14) so as to orient the quasi-totalityof an air flow issuing from said intake valve (10) directly toward thetransfer passageway (14).
 19. An engine according to claim 16,characterized in that a ratio between a maximum rise of said intakevalve (10) and an inside diameter of a seat (16) of said intake valve isgreater than 0.35.
 20. An engine according to claim 16, characterized inthat a head (10a) of said intake valve (10) cooperates, by its surface(21) remote form its seat (16), with a cavity (30) of conjugate shapeprovided in an opposed wall part of said precombustion chamber (13). 21.An engine according to claim 16, characterized in that the seat (16) ofsaid intake valve (10) is flat and tangent both to the wall of thetransfer passageway (14) and the surface of said cylinder (1).
 22. Anengine according to claim 16, characterized in that an intake pipe (11)for said intake valve is provided with a lip (33) immediately upstreamof the seat (16) of said intake valve and on a part of said pipe (11)closest to said transfer passageway.
 23. An engine according to claim16, characterized in that a fuel injector (120) opens onto theprecombustion camber (13) approximately on an axis of said intake valve(10) and on a side, of said precombustion chamber opposite to saidintake valve.
 24. An engine according to claim 16 wherein a stem of saidintake valve is located completely outside of an extension of thecylinder when said intake valve is closed.